The present invention relates generally to the field of reversible hydrogen storage. More particularly, the present invention relates to a dry homogenized metal hydrides, in particular aluminum hydride compounds, as a material for reversible hydrogen storage, and a method of making the same.
For decades, hydrogen has been targeted as the utopian fuel of the future due to its abundance and environmental friendliness. A major difficulty in the utilization of hydrogen as a fuel is the problem of onboard hydrogen storage. High pressure and cryogenic hydrogen storage systems are impractical for vehicular applications due to safety concerns and volumetric constraints. This has prompted an extensive effort to develop solid hydrogen storage systems for vehicular application. Metal hydrides, activated charcoal, and carbon nanotubules have been investigated as hydrogen carriers. For example, LaNiH5 has been investigated but has not proved satisfactory, due in part to its high cost. Unfortunately, despite decades of extensive effort, especially in the area of metal hydrides, no material has been found which has the combination of a high gravimetric hydrogen density, adequate hydrogen dissociation energetics, and low cost required for commercial vehicular applications.
It is known that the dehydrogenation of NaAlH4 is thermodynamically favorable at moderate temperatures. Dehydrogenation is known to occur by a multistep process involving the reactions as shown in equations 1and 2 below:
3 NaAlH4------ greater than Na3AlH6+2Al+3H2xe2x80x83xe2x80x83(1)
Na3AlH6------ greater than 3 NaH+Al+3/2H2xe2x80x83xe2x80x83(2)
The process is characterized by very slow kinetics and reversibility only under severe conditions. Thus, NaAlH4 has generally been precluded from consideration as a potential hydrogen storage material despite having a 5.6 weight percentage of hydrogen which is thermodynamically available at moderate temperatures. This thinking has been changed by the recent finding by Bogdanovic and Schwickardi that titanium doping of NaAlH4 enhances the kinetics of hydrogen desorption and renders the dehydriding process reversible under moderate conditions. Bogdanovic found that the onset of the initial dehydriding was lowered by about 50xc2x0 C. upon titanium wet doping by evaporation of an ether suspension of NaAlH4 which contained 2 mol % of titanium tetra-n-butoxide, Ti(OBun)4. This prior art approach however, is subject to many limitations. For example, the temperatures are still relatively high and the reaction kinetics are such that it does not produce a material suitable for practical vehicular applications.
Thus, further development of the kinetics of the dehydriding process is required to produce a material which is suitable for practical vehicular applications. It is of interest to investigate whether further developments in the kinetics of the reversible dehydriding of metallic hydrides, such as NaAlH4 and the like, can be achieved. Further, as the aforementioned discussion demonstrates, the need exists for safe, plentiful, low cost, and effective materials and methods for hydrogen storage and release.
The present invention provides novel reversible hydrogen storage materials and methods of making said materials, that are readily prepared from cheap, abundant starting materials.
More particularly, the present invention provides a new dry doping method comprising the steps of dry homogenizing metal hydrides by mechanical mixing, such as by crushing or ball milling a powder, of a metal aluminum hydride with a transition metal catalyst. The metal aluminum hydride is of the general formulas of: X1AlH4, where X1 is an alkali metal; X2(AlH4)2, where X2 is an alkaline earth metal; X3(AlH4)4, where X3 is Ti, Zr or Hf; X4AlH6, where X4 is an alkali metal; X5(AlH6)2, where X5 is an alkaline earth metal; X6(AlH6)4, where X6 is Ti, Zr or Hf; or any combination of the above hydrides.
In another aspect of the present invention, a material for storing and releasing hydrogen is provided, comprising a dry homogenized material having transition metal catalytic sites on a metal aluminum hydride compound, or mixtures of metal aluminum hydride compounds.
The inventors have found that the homogenization method of the present invention of metal aluminum hydrides with transition metal catalysts resulted in a lowering of the dehydriding temperature by as much as 75xc2x0 C. and markedly improves the cyclable hydrogen capacities. These findings represent a breakthrough in the application of this class of hydrides to hydrogen storage. In particular these findings enable the development of practical hydrogen storage materials and methods for the powering of vehicles, an achievement which has not before been realized.